Method of sampling underground formations



1964 J. P. GALLUS ETAL METHOD OF SAMPLING UNDERGROUND FORMATIONS 2Sheets-Sheet 1 Filed July 30, 1962 m \l w m w m m O JULIUS R GALLUSGERALD D. ORTLOFF BERTRAM T. WILLMAN INVENTORS C. 2 2 0 fin? ATT RNEY24, 1964 J. P. GALLUS ETAL 3,

METHOD OF SAMPLING UNDERGROUND FORMATIONS Filed July 30, 1962 2Sheets-Sheet 2 JULIUS P. GALLUS GERALD D. ORTLOFF BERTRAM T. WILLMANINVENTORS ATTORNEY United States Patent 3,1583%? li EETHGD 0F SAD HUNGUNDERGRQUNE) FGRMATIONS Julius P. Galius, Gerald D. Ortlolf, and BertramT. Willnian, Tulsa, Girla, amignors to Jersey Production ResearchCompany, a corporation of Belaware Filed .luly 30, 1962, Ser. No.213,349 7 Claims. (Cl. 175-59) The present invention is generallyconcerned with the sampling of underground formations. The inventionparticularly relates to a method for obtaining samples of subterraneanstrata from the bottoms of well bores that have been drilled into theearths crust in an effort to locate petroleum accumulations. It isespecially directed toward a method for obtaining samples of essentiallyunaltered fluid content by a technique which involves fragmentation of aportion of a stratum to be sampled, and which further involves the useof a non-invading fluid to seal the fragments obtained.

The stock tank barrel volumetric petroleum content of a porousunderground reservoir is normally calculated in terms of the reservoirvolume, the formation volume factor, the average reservoir porosity andthe average oil saturation within the porosity. In reservoirs wherein anumber of development wells have been drilled, the reservoir volume andthe formation volume factor can usually be determined with reasonableaccuracy by comparing structural maps and by measuring the gravity,temperature, pressure and gas content of the oil under reservoirconditions. The average porosity of the reservoir is generallydetermined by analyzing cores recovered from a number of developmentwells. Experience has demonstrated, however, that volumetric oil contentin the cores, as determined from core analysis, is usually not anaccurate indication of the actual quantity of oil present in areservoir. Reliable values for volumetric oil content, together with theother data, are extremely desirable to forecast the productive life ofoil and gas reservoirs, to select the primary recovery techniques mostsuitable for particular reservoirs, and to assess the susceptibility ofsuch reservoirs to later secondary and tertiary recovery processes. Itis a principal object of the invention to obtain samples from areservoir, the analysis of which will provide reliable values forvolumetric oil content.

Conventional core drilling systems utilize an annular bit and corebarrel Which are rotated from the earths surface by means of a rotarydrill string. A coring fluid is circulated downwardly through passagesin the drill string, barrel and bit, in order to maintain pressure onthe formation and thus prevent the escape of fluids contained therein.Cuttings produced by the bit are entrained in the coring fluid andreturned to the surface through the annulus surrounding the drillstring. As the bit cuts away the formation the central core whichremains is encased in the barrel. The core barrel is provided with meansfor breaking off the core after the barrel has been filled. Pressurecore barrels which can be sealed against changes in pressure are alsoused. After the core has been out the drill string is withdrawn from theborehole and the core is recovered.

It has been shown that the pressure maintained at the bottom of theborehole during a coring operation has a profound effect on the fluidcontent of the cores subsequently recovered. If this pressure is lessthan the formation pressure, fluids contained in the formation Will tendto flow out of the core into the borehole until equilibrium isestablished. If, on the other hand, the bottom hole pressure exceeds theformation pressure, the coring fluid will tend to flow into theinterstices of the formation and displace any oil, gas or watercontained ice therein. In either case the result is a change in thefluid content of a core such that subsequent measurements of the amountof fluid present will not accurately reflect the original fluid contentof the cores formation. Since this change in fluid content occurs as thecore is cut, the use of a pressure core barrel does not prevent it.

Several methods for avoiding the difficulty outlined above have beenproposed in the past. The most obvious of these involves carrying outcoring operations without any pressure differential between the core andfluid in the formation. This is impractical, if not impossible, becausethe formation pressure cannot be con veniently measured during coredrilling, and moreover, because the fluid pressure cannot be controlledwith sufficient accuracy. The use during rotary core drilling of adrilling fluid which will not invade a formation under pressuresconsiderably in excess of the formation pressure has been disclosed, butefforts to develop an ideal fluid have not been entirely successful.Systems which include the use of radioactive tracers to permit adetermination of the extent to which core invasion has occurred andsystems for freezing the core in situ have been proposed but have notbeen found to be generally effective.

Non-invading core drilling fluids which have been used or suggested foruse include various polymeric materials such as silicones andpolyesters. Alkyd resins such as the reaction product of ethylene glycoland maleic anhydride; condensation products such as cresol-formaldehyderesins; mercury, molten metal and low melting metal alloys such asalloys of lead, bismuth, tin and the like have also been suggested.

The most promising of the non-invading coring fluids which have beendeveloped are polymeric elastomer latices, dispersions or suspensions.These materials are aqueous dispersions of oil-resistant polymericelastomers, including natural and synthetic rubber latices, both virginand reclaimed. Mixtures of natural and synthetic latices have also beenfound suitable. Preferred latices may be derived from syntheticelastomers prepared by the polymerization of olefinically unsaturatedhydrocarbons, or by the copolymerization of such hydrocarbons with otherolefinically unsaturated monomers, The olefinically unsaturatedhydrocarbons include oleflns such as isobutylene and the pentylenes;diolefins such as butadiene, isoprene, piperylene, dimethyl butadieneand 2- methyl pentadiene; and vinyl aromatics such as styrene, methylstyrene and vinyl toluene. Mixtures of two or more of such hydrocarbonshave also been found suitable. Olefinically unsaturated monomers whichmay be copolymerized with the hydrocarbons include halogenatedolefinically unsaturated compounds such as vinyl chloride,allyl-chloride and chloroprene, unsaturated ester such as vinyl acetate,allyl propionate, methyl methacrylate, ethyl acrylate, methyl fumerate,ethyl maleate and propyl itaconate. Unsaturated nitriles such asacrylonitrile, methacrylonitrile, ethyl acrylonitrile andchloroacrylonitrile. Unsaturated ketones such as methyl vinyl ketones,cyclic vinyl compounds such as vinyl pyridine, and mixtures thereof. Itwill be recognized that not all of these elastomer-s are equallyeifective for use in preparing a non-invading coring fluid.

Specific examples of elastomers prepared from the foregoing monomerswhich have been found suitable in the form of latices for use asnon-invading fluids include polyisobutylene, polystyrene, polybutadiene,polyisoprene, butadiene-isoprene copolymers, isoprene-isobutylenecopolymers, isobutylene-styrene copolymers, piperylene-vinyl acetatecopolymers, butadiene-styrenevinyl chloride copolymers,butadiene-acrylonitrile copolymers, butadiene-methacrylonitrilecopolymers and iso prene-chloroprene-vinyl acetate copolymers,

A wide range of concentrations of dispersed clastomer in the latices isuseful for the purposes of the invention. Latices containing from aboutto about 75% elastomer are generally suitable. Those containing fromabout to about 7(;% by weight are preferred.

It has now been found that rotary core rilling with the use of the abovenon-invading latex fluids is not a perfect solution to the problem ofhow to recover at the surface of the earth a sample of subterraneanreservoir rock, unaltered with respect to its original fluid content.Laboratory tests have shown that the static exposure of a reservoir rocksurface to any of the above latices, under substantial pressures, doesnot result in significant invasion of tne rock. Eowever, attempts tosecure uninvaded cores by rotary coring, with the circulation of thesame latices as drilling fluids, have shown that the static tests arenot a satisfactory indication of the latex behavior in the dynamicenvironment of rotary core drilling conditions. The difficulty is notcaused by any invasion of the core by dispersed elastomer particles, butby filtrate invasion, which is a leakage of the aqueous portion of thelatex through the filrn of solids deposited on the rock surface.

Referring now to FIGURE 1, the results of a typical static invasion testare shown as a plot of filtrate invasion, measured in cubic centimetersof filtrate per square centimeter of reel; surface exposed to the lluid,versus the total time of exposure. The principal significance of thetest is that it shows about 76% of the ten-minute invasion total to haveoccurred in less than one second of exposure. But the total invasion ofabout 0.914 :c./cm. at the end of ten minutes is still negligible.Therefore, the initial spurt loss shown by FIGURE 1 can nevertheless betolerated, as such, if an essentially static exposure of the sample ismaintained.

The present invention is based primarily upon the discovery thatfiltrate invasion during conventional rotary coring operations isgreatly increased by a continuous or repeated removal of the impermeablefilm ahead of the rotary core bit as coring proceeds. The effectproduced is a spurt loss repeated with each cut of a tooth. Accordingly,it is an object of this invention to avoid the detrimental effectattributed to rotary core drilling in connection with the use of anon-invading fluid by providing a sampling technique which involves avirtually instantaneous separation of samples from the formation, suchthat the fragments obtained are immersed in the non-invading fluid underessentially static conditions whereby any significant invarsion isavoided.

Broadly, the invention is a method for recovering a sample ofessentially unaltered fluid content from a subterranean stratumpenetrated by a borehole which comprises at least partially filling saidborehole with a non invading fluid, fragmentizing a region of saidstratum adjacent the bottom of said borehole, whereby the resultingfragments are sealed by exposure to said non-invading fluid, and thenraising the sealed fragments to the sur-, face while maintaining apressure thereon which is not substantially less than the naturalpressure of said stratum.

The preferred embodiment of the invention comprises the steps ofdrilling a borehole in the vicinity of the stratum to be sampled,filling at least a portion of said borehole with a non-invading fluid,introducing an explosive charge into the borehole, firing said charge ator near the bottom of the borehole, said charge being of sufficientmagnitude to fragmentize a region of said stratum adjacent theexplosion, whereby the resulting fragments are sealed by exposure toone-invading fluid, and thereafter raising the sealed fragments to thesurface while maintaining apressure thereon which is not substantiailyless than the natural pressure of said stratum.

The method of the present invention may be employed to obtain samplesfrom the bottoms or'the side walls of existing wells, or it may beemployed to obtain samples by drilling a new borehole from the earthssurface down to a given substratum the sampling of which is desired.

Once a borehole has been selected or drilled, the next step of theinvention is to fill the borehole with suflicient non-invad ng fluid toprevent the flow of fluid from the- When the preferred non-invadinglatex fluids mentioned above are used, a considerably greaterhydrostaitc head may be tolerated.

The next step involves a fragmentation of the formation at the bottom ofthe borehole in order to produce fragments which are suitable andofadequate size for analysis as samples when retrieved at the surface ofthe earth. Suitable fragments may be made by using a chip-coring tool atthe end of a wire line or mounted on the end of a tubing string in orderto break chips from the formation at the bottom of the borehole. Asuitable example of this technique is found in US. 2,819,- 03-3. Anyconventional drilling tool of a reciprocating or percussive design isuseful for the purposes of the present invention, provided that norotary grinding mechanism is obtained.

The preferred method of fragmentizing the bottom of the borehole is tointroduce and discharge or detonate an explosive at the bottom of theborehole. Essentially any conventional explosive charge may be employedin accordance with the invention, examples of which include commercialdynamite, nitroglycerin and TNT. The quantity of explosive required tocreate optimum fragmentation varies over a wide range. In the case ofnitroglycerin, for example, the amount required will vary from one quartto one hundred quarts or more depending upon the character of the rockand upon the extent of fragmentation desired. Equivalent amounts ofdynamite or TNT are also useful.

Suitable techniques for placing and detonating an explosive charge in aborehole are well known, especially in the art of completing wells. Ithas been a common practice, for example, when shooting a well to pack asubstantial portion of the borehole above the charge with gravel, sandor other material, in order to concentrate the effects of the blast atthe desired level. Similarly, gravel packing will be desirable in someinstances when sampling a formation in accordance with the presentinvention.

The fragments created by the blast, or other technique, are immediatelysealed by immersion in the non-invading fluid standing, in the wellbore.In accordance with one embodiment of the invention, a pressure corebarrel is then lowered into the borehole to gather the sealed fragmentsand bring them to the surface without changing the original-pressure.The pressure barrel and contents then be frozen, with or without firstreplacing some of the fluid with nitrogen gas under pressure. The frozensamples are then ready for shipment to the laboratory for analysis.

An important feature of the invention is that an explosion of sufficientmagnitude to fragmentize the bottomhole rock will not at the same timecause significant invasion of the fragments produced, and thereby defeatthe essential object of the invention. Calculations show that theexposure of a typical rock surface to a blast pressure differential of1,000 atmospheres results In only negligible invasion, because of theextremely short duration of the blast. This is true, even beforetakinginto account the added resistance to invasion provided by thesealing action of the borehole fluid.

Moreover, thefragments produced from rock lying immediately adjacent theexplosion, and therefore most affected by the blast, are of relativelylittle interest any- Way.. The larger fragments created come from pointswithin the formation relatively'more remote from the center of theblast, and are therefore less disturbed by itand by the drilling fluidused to sink the hole. The force of the blast is transmitted to theselatter regions of the formation primarily by shock waves travelingthrough the rock, rather than by direct exposure to the high pressuregases released in the explosion. This further lessens the chance ofinvasion. Accordingly, it becomes clear that the larger fragments notonly permit a more accurate analysis because of their size alone, butare also inherently more representative of the undisturbed formation.

In accordance with another embodiment of the invention, the sealedfragments are lifted to the surface with the aid of an ordinary bailer.In order to retrieve the samples by this technique, it is necessary tomaintain surface pressure control in the wellbore during the bailingoperation in order to avoid a bleeding of the samples as they arebrought to the surface.

A lubricator is attached to the well casing at the surface, and thebailer is run on a cable or wire line through the lubricator anddownhole to pick up the fragment samples. As the bailer is lifted, thepressure maintained in the lubricator must be at least as great as thenatural pressure of the formation from which the sample fragments weretaken. The bailer is pulled into the lubricator, the lubricator valvesare closed, and the lubricator and its contents are frozen to permitremoval of the samples without bleeding them.

Referring now in detail to FIGURE 2, borehole 11 has been drilled intoformation 12 to be sampled. Shaded area 13 indicates the zone ofdrilling fluid invasion into the formation while drilling. Dotted line14 indicates the position of the borehole wall prior to the detonationof an explosive charge in accordance with the preferred embodiment ofthe invention. Fragments 16 and 17 created by the blast are dislodgedfrom the borehole wall as well as from the zone immediately below theoriginal borehole bottom. Thus the borehole is enlarged in the region ofthe blast as indicated by numeral 15. This caving of fragments from theborehole wall creates a mound of debris 16. The larger and more usefulfragments are created below the borehole bottom and are designated bynumeral 17.

When bailing out the debris and fragments from the borehole it becomesapparent that the smaller fragments produced from rock lying immediatelyadjacent the explosion will be relatively more affected by the last thanwill the larger fragments 17. It is important to observe that fragments17 were outside the region of original fluid invasion 13 and were alsoout of contact with drilling fluid at the time of the explosion. Largerfragments 17 are of course sealed subsequent to the blast by thenoninvading fluid standing in the borehole. Thus, as pointed outearlier, the larger fragments not only permit a more accurate analysisbecause of their size alone, but are also inherently more representativeof the undisturbed formation.

What is claimed is:

1. A method for recovering a sample of essentially unaltered fluidcontent from a subterranean stratum, which comprises drilling a boreholeinto the vicinity of the stratum to be sampled, at least partiallyfilling said borehole with a non-invading fluid, firing an explosivecharge near the bottom of said borehole, said charge being of sufiicientmagnitude to fragmentize a region of said stratum adjacent theexplosion, whereby the resulting fragments are sealed by exposure tosaid non-invading fluid, and then raising the sealed fragments to thesurface while maintaining a pressure thereon which is not substantiallyless than the natural pressure of said stratum.

2. A method as defined by claim 1 wherein said noninvading fluidcomprises an oil-resistant polymeric elastomer suspension.

3. A method for recovering a sample of essentially unaltered fluidcontent from a subterranean stratum penetrated by a borehole whichcomprises at least partially filling said borehole with a non-invadingfluid, firing an explosive charge within said borehole near that portionof the stratum to be sampled, whereby the resulting fragments are sealedby exposure to said fluid, and then raising at least one of saidfragments to the earths surface while maintaining a pressure thereonwhich is not substantially less than the natural pressure of saidstratum.

4. A method for recovering a sample of essentially unaltered fluidcontent from a subterranean stratum penetrated by a borehole whichcomprises at least partially filling said borehole with a non-invadingfluid, fragmentizing a region of said stratum adjacent said borehole,whereby the resulting fragments are sealed by exposure to said fluid,and then lifting at least one of said fragments to the earths surface ina bailer, while gradually increasing the pressure maintained at thewellhead to a maximum pressure at least as great as the natural pressureof the sampled stratum.

5. A method for recovering a sample of essentially unaltered fluidcontent from a subterranean stratum penetrated by a borehole whichcomprises at least partially filling said borehole with a non-invadingfluid, substantially instantaneously separating a plurality of fragmentsfrom a region of said stratum adjacent said borehole, whereby theresulting fragments are immediately sealed by exposure to said fluid,and then raising at least one of said fragments to the earths surfacewhile maintaining a pressure thereon which is not substantially lessthan the natural pressure of said stratum.

6. A method as defined by claim 5, wherein said step of substantiallyinstantaneously separating a plurality of fragments from said stratum isaccomplished by firing an explosive charge within said borehole nearthat portion of the stratum to be sampled.

7. A method as defined by claim 5, wherein said step of substantiallyinstantaneously separating a plurality of fragments from a region ofsaid stratum adjacent said borehole is accomplished by mechanicalimpact.

References Cited in the file of this patent UNITED STATES PATENTS2,146,263 Johnston Feb. 7, 1939 2,264,449 Mounce Dec. 2, 1941 2,373,323Macready Apr. 10, 1945 2,880,969 Williams Apr. 7, 1959 3,064,742Bridwell Nov. 20, 1962 3,112,799 Gallus Dec. 3, 1963

5. A METHOD FOR RECOVERING A SAMPLE OF ESSENTIALLY UNALTERED FLUIDCONTENT FROM A SUBTERRANEAN STRATUM PENETRATED BY A BOREHOLE WHICHCOMPRISES AT LEAST PARTIALLY FILLING SAID BOREHOLE WITH A NON-INVADINGFLUID, SUBSTANTIALLY INSTANTANEOUSLY SEPARATING A PLURALITY OF FRAGMENTSFROM A REGION OF SAID STRATUM ADJACENT SAID BOREHOLE, WHEREBY THERESULTING FRAGMENTS ARE IMMEDIATELY SEALED BY EXPOSURE TO SAID FLUID,AND THEN RAISING AT LEAST ONE OF SAID FRAGMENTS TO THE EARTH''S SURFACEWHILE MAINTAINING A PRESSURE THEREON WHICH IS NOT SUBSTANTIALLY LESSTHAN THE NATURAL PRESSURE OF SAID STRATUM.